Polyesters can be modified during their preparation with polyfunctional modifiers to obtain branched polyesters having particular chemical or physical properties. These polyfunctional modifiers are also known as branching agents. These modifiers generally have different molecular weights, volatilities, chemical reactivities and other properties. In the prior art, when a polyester is modified with a polyfunctional modifier to form a branched polyester, it appears that no attempt is made to insure that all of the modifier reacts to the same degree each time a batch of polyester is made. As a result, the amount of modifier incorporated into the branched polyester varies from batch to batch.
Branched polyesters are known to be useful in a variety of products. However, practical manufacture of branched polyesters has been hindered because it is difficult to consistently reproduce such polyesters from one batch to another using prior art methods. Prior art methods do produce ways for preparing branched polyesters having the same or similar inherent viscosities. However, branched polyesters having the same or similar inherent viscosities do not necessarily have the same physical or chemical properties. For example, I have observed that two branched polyesters which are prepared from the same materials and have the same inherent viscosity can have such different physical and chemical properties, due to different amounts of branching, that one polyester is suitable for use as a film support while the other is not.
It is well known that the inherent viscosity actually measured for a branched polyester is really an "overall" inherent viscosity. Such "overall" viscosity is the sum of an inherent viscosity due to the linear portion of the polyester plus an inherent viscosity due to branched portions of the polyester. In a method for reproducibly preparing branched polyesters, the "overall" inherent viscosity at a given time in the preparation contains a component that is due solely to the branched portions of the polyester and is substantially the same from batch to batch at such given time.
Methods for reproducibly preparing branched polyesters have eluded workers in the art because of the inherent difficulty encountered in controlling the competing rates of linear extension and branching during the formation of such branched polyesters. Typically, for polyesters useful as films or fibers, the rate of linear extension during polycondensation is significantly greater than the rate of branching. When polycondensation is carried out under conventional conditions, that is, under conditions which "drive" the reaction to completion, this difference in rates is so great that little or no branching occurs. In general, the prior art methods compensate for this effect by using high concentrations of polyfunctional compounds for preparing branched polyesters. However, the use of high concentrations of polyfunctional compounds often results in undesirable gelation or crosslinking of the polyester.
Branched polyesters and methods for making them are known, as disclosed for example, in U.S. Pat. No. 3,576,773 (issued Apr. 27, 1971 to Vaginay) and British Pat. No. 1,027,613 (published Apr. 27, 1966). Typically, such methods use polyfunctional compounds which have three or more reactant functional radicals, such as hydroxyl and carboxyl radicals. These polyfunctional compounds are also known as branching agents. U.S. Pat. No. 4,013,624 (issued Mar. 22, 1977 to Hoeschele) relates to the preparation of high molecular weight branched copolyesters using a "critical" concentration of branching agent. This reference suggests that use of this concentration provides high molecular weight polyesters in the shortest possible time. But like the rest of the prior art, it lacks any teaching or suggestion of controlling polycondensation conditions in order to prepare branched polyesters reproducibly.
Hence, it would be desirable to have a method of preparing branched polyesters whereby one could practically reproduce branched polyesters having a pre-selected combination of properties. Further, it would be desirable to have such a method wherein the concentration of polyfunctional modifier would be varied in a wide range without fear of undesired gelation or crosslinking.